It has been stressed (Wielen 1997; Wielen et al. 1999) that the proper motions provided by the Hipparcos catalogue may be systematically in error for long-period binaries (i.e. with P > 3 yr), if those were not recognized as such by the reduction consortia. The orbital motion may then add to the actual proper motion, changing both its direction and modulus. The present sample offers a good opportunity to evaluate the impact of this effect.
Figures 7 and 8 compare the position angle and
modulus,
respectively, of the Hipparcos proper motion with those derived when account is
made of the orbital motion as in the present work. It is clearly apparent
that the direction of the proper motion listed in the Hipparcos catalogue is
correct, as expected, for orbits with periods less than 3 yr, corresponding to
the duration of the Hipparcos mission. The Hipparcos values are the less
accurate in the orbital-period range 3 to about 5 yr. The position angle
quoted by the Hipparcos catalogue may then be off by several dozens
degrees
(a good illustration of this situation is offered by the the strong Ba
star HIP 110108 in Table 2),
though it differs generally by less than
from the correct value.
For longer orbital periods, the orbital motion becomes negligible over the
duration of the Hipparcos mission, and the proper motions are again rather
well determined in the Hipparcos catalogue. The situation is almost
identical for the the proper-motion modulus, except that the error bars
now become very large for orbital periods longer than 3 yr. This situation
translates the fact that the proper motion and the semi-major axis are
strongly correlated (resulting in a large formal uncertainty on the proper
motion), because the two motions are difficult to disentangle when the
Hipparcos data sampled only a fraction of the orbit. This situation is
encountered e.g. for HIP 104785 and 104732 (whose astrometric orbits
have not been retained precisely because of the large uncertainty on
a0 introduced by the strong correlation with
),
and results in sometime large
differences between the a0 and
values derived from the NDAC, FAST
and NDAC+FAST data sets, since they are now very sensitive to the measurement
errors.
Figure 9 compares the parallax listed in the Hipparcos catalogue with that derived in the present work, as a function of the orbital period. It turns out that the agreement is good, except for systems with orbital periods close to 1 yr. For those systems, the parallax cannot be accurately derived since the orbital motion and the parallactic motion are strongly entangled. This situation is encountered for HIP 17402, 53763 and 62827. Large error bars at periods different from 1 yr correspond to systems with parallaxes smaller than 3 mas.
A few stars (HIP 8876, 29740, 29099, 32831, 36613) in our sample had negative
parallaxes in the Hipparcos catalogue. The parallaxes we derive for these
systems are slightly positive (
was adopted to guarantee that
property), but the associated error bar encompasses zero, so
that these new parallaxes are just useless.
Copyright The European Southern Observatory (ESO)